Metabolism is the complete set of chemical reactions that occurs in living cells. These processes are the basis of life, allowing cells to grow and reproduce, maintain their structures, and respond to their environments.

There are two kinds of Metabolism:

• Catabolism: Catabolic reactions (performed by the cell) yield energy, by breaking down food that we eat or tissue (like muscle and fat) that we have.
• Anabolism: Anabolic reactions (performed by the cell), on the other hand, use this energy to construct components of cells such as proteins and nucleic acids that form in turn tissue known as muscle, fat, skin, bone.

Most of the structures that make up animals, plants and microbes are made from three basic classes of molecule: amino acids, carbohydrates and lipids (often called fats).

For humans there are 6 major types of nutrition (types of nutrition molecules)"

• macro-nutrients
  • water
  • carbohydrates
  • protein (amino-acids)
  • fats (lipids)
• micro-nutrients
  • minerals
  • vitamins

The major food molecule in living organisms is a sugar called glucose. Most carbohydrates (sugars and starches) are converted into glucose before they are broken down to release energy. The series of steps where glucose is broken down to release energy begins with a metabolic pathway called glycolysis. Glycolysis is the "lysing" or cutting of glucose to release energy. The six carbon sugar, glucose, is cut in half and converted into two three carbon sugars called pyruvate. What happens next depends on the presence or absence of oxygen.

If oxygen is present, then glucose can be broken all the way down into carbon dioxide and water. This process is called aerobic respiration because it requires air (oxygen). In the absence of oxygen, the cell uses a process called anaerobic fermentation. or simply fermentation. Fermentation doesn't break the sugar down any further, it simply helps reset the system so that more sugar can be broken down.

Because aerobic respiration breaks the sugar all the way down, it releases much more energy than fermentation.

About the major Nutrients

Carbohydrates

Carbohydrates are the most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals). The basic carbohydrate units are called monosaccharides and include galactose, fructose, and most importantly glucose.

Carbohydrate molecules consist of carbon, hydrogen, and oxygen. They have a general formula Cn(H2O)n. There are several sub-families based on molecular size.

Carbohydrates are chemical compounds that contain oxygen, hydrogen, and carbon atoms, and no other elements. They consist of monosaccharide sugars of varying chain lengths.

Certain carbohydrates are an important storage and transport form of energy in most organisms, including plants and animals. Carbohydrates are classified by their number of sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).

Proteins

Proteins are made of amino acids arranged in a linear chain and joined together by peptide bonds. Many proteins are the enzymes that catalyze the chemical reactions in metabolism. Other proteins have structural or mechanical functions, such as the proteins that are used in building muscle.

All proteins contain carbon, hydrogen, oxygen and nitrogen. Some also contain phosphorus and sulfur. The building blocks of proteins are amino acids. There are 20 different kinds of amino acids used by the human body. They unite by peptide bonds to form long molecules called polypeptides. Polypeptides are assembled into proteins. Proteins have four levels of structure

Primary
Primary structure is the sequence of amino acids bonded in the polypeptide.

Secondary
The secondary structure is formed by hydrogen bonds between amino acids. The polypeptide can coil into a helix or form a pleated sheet.

Tertiary
The tertiary structure refers to the three-dimensional folding of the helix or pleated sheet.

Quaternary
The quaternary structure refers to the spatial relationship among the polypeptide in the protein.

Enzymes

Enzymes are essential for life because most chemical reactions in living cells would occur too slowly or would lead to different products without enzymes. A biological molecule that catalyzes a chemical reaction. Most enzymes are proteins and the word "enzyme" is often used to mean a protein enzyme. Some RNA molecules also have a catalytic activity, and to differentiate them from protein enzymes, they are referred to as RNA enzymes or ribozymes.

Lipids

Lipids (fats) are the most diverse group of biochemicals. Their main structural uses are as part of biological membranes such as the cell membrane, or as a source of energy.

Minerals

Inorganic elements (minerals) play critical roles in metabolism; some are abundant (e.g. sodium and potassium) while others function at minute concentrations. About 99% of mammals' mass are the elements carbon, nitrogen, calcium, sodium, chlorine, potassium, hydrogen, oxygen and sulfur.

The organic compounds (proteins, lipids and carbohydrates) contain the majority of the carbon and nitrogen and most of the oxygen and hydrogen is present as water.

The inorganic elements act as ionic electrolytes.

Energy Molecules

Metabolism is basically about the production of energy from the breaking down of certain complex molecules (Catabolism) and the storage of the produced energy as body tissue that is basically another set of complex molecules (Anabolism).

In this process, energy rich molecules play an important role. Major energy rich molecules are the following:

• Adenosine Triphosphate (ATP): ATP is the currency of the cell. When the cell needs to use energy such as when it needs to move substances across the cell membrane via the active transport system, it "pays" with molecules of ATP. ATP cannot be stored, hence its consumption must closely follow its synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body. Looking at it another way, a single cell uses about 10 million ATP molecules per second to meet its metabolic needs, and recycles all of its ATP molecules about every 20-30 seconds.
• Flavin Adenine Dinucleotide (FAD): When two hydrogen atoms are bonded, FAD is reduced to FADH2 and is turned into an energy-carrying molecule. FAD accommodates two equivalents of Hydrogen; both the hydride and the proton ions. This is used by organisms to carry out energy requiring processes. FAD is reduced in the citric acid cycle during aerobic respiration.
• Nicotinamide Adenine Dinucleotide (NADH): Nicotinamide adenine dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP) are two important cofactors found in cells. NADH is the reduced form of NAD+, and NAD+ is the oxidized form of NADH.

  The reducing potential stored in NADH can be converted to ATP through the electron transport chain or used for anabolic metabolism. ATP "energy" is necessary for an organism to live. Green plants obtain ATP through photosynthesis, while other organisms obtain it by cellular respiration.

  Nicotinamide adenine dinucleotide phosphate (NADP+)NADP is used in anabolic reactions, such as fat acid and nucleic acid synthesis, that require NADPH as a reducing agent.

Celullar Respiration

Cellular respiration is the energy releasing process by which sugar molecules are broken down by a series of reactions and the chemical energy gets converted to energy stored in ATP molecules.

The reactions that convert the fuel (glucose) to usable energy (ATP) are collectivelly referred to as "cellular respiration" or "aerobic respiration" and they are:

• glycolysis
• Krebs cycle (sometimes called the citric acid cycle)
• the electron transport chain

Oxygen is needed as the final electron acceptor, and carrying out cellular respiration is the very reason we breathe and the reason we eat.

Glycolysis

The glycolytic pathway (glycolysis) is where glucose, the smallest molecule that a carbohydrate can be broken into during digestion, gets oxidized and broken into two 3-carbon molecules (pyruvates), which are then fed into the Kreb's Cycle. Glycolysis is the beginning of cellular respiration and takes place in the cytoplasm. Metabolism also makes use of proteins as catalysts. Oxygen is also needed for the oxidization that occurs.

Krebs Cycle

The Krebs cycle was named after Sir Hans Krebs (1900-1981), who proposed the key elements of this pathway in 1937 and was awarded the Nobel Prize in Medicine for its discovery in 1953.

All cells must produce energy to survive. Hans A. Krebs first elucidated the process of cells converting food into energy, the Citric Acid Cycle, in 1937. Krebs proposed a specific metabolic pathway within the cells to account for the oxidation of the basic components of food — carbohydrates, protein and fats — for energy. The Krebs’ cycle takes place inside the mitochondria or 'power plant' of cells and provides energy required for the organism to function.

Two molecules of pyruvate enter the Krebs cycle, which is called the aerobic pathway because it requires the presence of oxygen in order to occur. This cycle is a major biological pathway that occurs in humans and every plant and animal.

After glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the interior of the mitochondrion. Once the pyruvic acid is inside, carbon dioxide is enzymatically removed from each three-carbon pyruvic acid molecule to form acetic acid. The enzyme then combines the acetic acid with an enzyme, coenzyme A, to produce acetyl coenzyme A, also known as acetyl CoA.

Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split into eight steps. See diagram below.

The Krebs Cycle steps

Step 1 The acetic acid subunit of acetyl CoA is combined with oxaloacetate to form a molecule of citrate. Acetyl coenzyme A acts only as a transporter of acetic acid from one enzyme to another. After Step 1, the coenzyme is released by hydrolysis to combine with another acetic acid molecule and begin the Krebs’ Cycle again.

Step 2 The citric acid molecule undergoes an isomerization. A hydroxyl group and a hydrogen molecule are removed from the citrate structure in the form of water. The two carbons form a double bond until the water molecule is added back. Only now, the hydroxyl group and hydrogen molecule are reversed with respect to the original structure of the citrate molecule. Thus, isocitrate is formed.

Step 3 The isocitrate molecule is oxidized by a NAD molecule. The NAD molecule is then reduced by the hydrogen atom and the hydroxyl group. The NAD binds with a hydrogen atom and carries off the other hydrogen atom leaving a carbonyl group. This structure is very unstable, so a molecule of CO2 is released, creating alpha-ketoglutarate.

Step 4 In this step, coenzyme A, returns to oxidize alpha-ketoglutarate. A molecule of NAD is reduced again to form NADH and leaves with another hydrogen. This instability causes a carbonyl group to be released as carbon dioxide and a thioester bond is formed in its place between the former alpha-ketoglutarate and coenzyme A to create a molecule of succinyl-coenzyme A complex.

Step 5 A water molecule sheds its hydrogen atoms to coenzyme A. Then, a free-floating phosphate group displaces coenzyme A and forms a bond with the succinyl complex. The phosphate is then transferred to a molecule of ADP to produce an energy molecule of ATP. It leaves behind a molecule of succinate.

Step 6 In this step, succinate is oxidized by a molecule of FAD (Flavin Adenine Dinucleotide). The FAD removes two hydrogen atoms from the succinate and forms a double bond between the two carbon atoms to create fumarate.

Step 7 An enzyme adds water to the fumarate molecule to form malate. The malate is created by adding one hydrogen atom to a carbon atom and then adding a hydroxyl group to a carbon next to a terminal carbonyl group.

Step 8 In this final step, the malate molecule is oxidized by a NAD molecule. The carbon that carried the hydroxyl group is now converted into a carbonyl group. The end product is oxaloacetate which can then combine with acetyl-coenzyme A and begin the Krebs’ Cycle all over again.

Electron Transport System

This is the most complicated system of all. In the respiration chain, oxidation and reduction reactions occur repeatedly as a way of transporting energy. The respiratory chain is also called the electron transport chain. At the end of the chain, oxygen accepts the electron and water is produced.

Redox Reaction

This is a simultaneous oxidation-reduction process whereby cellular metabolism occurs, such as the oxidation of sugar in the human body, through a series of very complex electron transfer processes.

Anaerobic Respiration

Two molecules of ATP are required for glycolysis, but four are produced (at the end of the whole metabolism process) so there is a net gain of two ATP per glucose molecule.

Two NADH molecules transfer electrons (in the form of hydrogen ions) to the electron transport chain in the mitochondria, where they will be used to generate additional ATP.

During physical exertion when the mitochondria are already producing the maximum ATP possible with the amount of oxygen available, glycolysis can continue to produce an additional 2 ATP per glucose molecule without sending the electrons to the mitochondria.

However, during this anaerobic respiration lactic acid is produced, which may accumulate and lead to temporary muscle cramping.

Catabolism

Catabolism is basically what we have already described as "cellular respiration". However, there is one more very important related aspect: Breaking down of fat and breaking down of muscle when nutrient intake is not enough.

Glycogen breakdown

See "Glycolysis": http://en.wikipedia.org/wiki/Glycolysis

fat breakdown (fat catabolism)

http://www.biocrawler.com/encyclopedia/Fat_catabolism
also see "Lipolysis":
* http://en.wikipedia.org/wiki/Lipolysis

The breakdown of fat stored in fat cells is known as lipolysis. During this process, free fatty acids are released into the bloodstream and circulate throughout the body. Ketones are produced, and are found in large quantities in ketosis (an adaptive metabolic state that occurs when insufficient carbohydrates are present in the diet). Lipolysis testing strips such as Ketostix are used to recognize ketosis.

The following hormones induce lipolysis: epinephrine, norepinephrine, glucagon and adrenocorticotropic hormone. These trigger 7TM receptors, which activate adenylate cyclase. This results in increased production of cAMP, which activates protein kinase A, which subsequently activate lipases found in adipose tissue.

Triglycerides undergo lipolysis (hydrolysis by lipases) and are broken down into glycerol and fatty acids. Once released into the blood, the relatively hydrophobic free fatty acids bind to serum albumin for transport to tissues that require energy. The glycerol also enters the bloodstream and is absorbed by the liver or kidney where it is converted to glycerol 3-phosphate by the enzyme glycerol kinase. Hepatic glycerol 3-phosphate is mostly converted into Dihydroxyacetone (DHAP) and then glyceraldehyde 3-phosphate (G3P) to rejoin the glycolysis and gluconeogenesis pathway.

muscle breakdown (protein catabolism)

http://www.biocrawler.com/encyclopedia/Protein_catabolism

Anabolism

Short term energy storage (sugar - glycogen = storage form of glucose)

The storage capacity for carbohydrate in the human body is quite limited. An average human's muscles can sock away about 300 to 400 grams of the stuff. The liver can put up to 90 grams in its biscuit tin. Carbohydrate storage depots in the rest of the body are negligible, so the grand total checks in at less than 2000 stored calories. That's the best we can do!
As Sears puts it, once those limited storage areas are filled to capacity, there's just one area that incoming carbohydrate can go: it must be pushed into your belly, buttocks, or thighs as plain-old fat.

Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides.

Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack the ability to pass glucose into the blood, so the glycogen they store internally is destined for internal use and is not shared with other cells, unlike liver cells.

Due to the body's inability to hold more than around 2,000 kcal of glycogen, long-distance athletes such as marathon runners, cross-country skiers, and bicycle racers go into glycogen debt, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without enough energy consumption. This phenomenon is referred to as "hitting the wall" or "bonking". In marathon runners it normally happens around the 20 mile (32 km) point of a marathon, where around 100 kcal are spent per mile, depending on the size of the runner and the race course. When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move.

Long term energy storage (fat buildup or fat anabolism)

Triglycerids are the main storage form of fat.
http://www.biocrawler.com/encyclopedia/Fat_anabolism
Also see "Lipogenesis": http://en.wikipedia.org/wiki/Lipogenesis

Muscle build-up

Having enough protein intake ensures that minimal muscle tissue is lost (catabolised). Exercizing while providing enough protein intake will increase muscle tissue mass. Adequate carbohydrate intake is also important for optimal muscle growth.